1. Technical Field
The present invention relates to an improvement of a cutting technology for fusing glass sheets.
2. Background Art
Conventionally, as a glass sheet cutting method, there have been employed a method in which a glass sheet is cleaved in such a manner that a scribe line is formed in a surface of the glass sheet and then a bending stress is applied to the scribe line (cleaving using a bending stress), and laser cleaving in which a glass sheet is cleaved in such a manner that an initial crack is formed in the glass sheet and then the initial crack is propagated by irradiation heat of a laser beam (cleaving using a thermal stress).
However, the cleaving using a bending stress has a problem in that it is impossible to avoid occurrence of minute glass powder and to easily remove the minute glass powder even in washing performed after cutting. This problem is critical particularly in a glass substrate for use in a display and the like that are required to have a high level of cleanness. Further, in the cleaving using a bending stress, a cut end portion of the glass sheet has an angular shape, and hence a defect such as a chip easily occurs. Accordingly, it is necessary to perform chamfering on the cut end portion of the glass sheet after cutting.
On the other hand, in the laser cleaving, it is possible to cleave the glass sheet with little defect, but it is extremely difficult to avoid contact between cut end surfaces of the glass sheet when dividing the glass sheet into cleaved pieces. Accordingly, at the time of the dividing, due to rubbing between the cut end surfaces of the glass sheet or the like, a minute defect may be formed in the cut end surfaces. Further, even in the laser cleaving, similarly to the above-mentioned cleaving using a bending stress, the cut end portion of the glass sheet has an angular shape, and hence it is necessary to perform chamfering after cutting.
As a cutting method capable of coping with the above-mentioned problems, laser fusing attracts attention.
The laser fusing is a method of cutting a glass sheet while melting and removing a part of a glass substrate with irradiation heat of a laser beam. Accordingly, the laser fusing can prevent occurrence of glass powder. Further, an unnecessary part of glass is molten and removed, and thus a predetermined clearance is formed between fused end surfaces (cut end surfaces). With this, it is possible to reliably avoid such a situation that the fused end surfaces of the glass sheet come into contact with each other at the time of dividing. In addition, the fused end surfaces are molten and formed into smooth fire-polished surfaces, and hence even without performing chamfering on the fused end surfaces separately, the glass sheet is less likely to be damaged.
However, even the laser fusing still has a problem in actual use.
That is, fusing of the glass sheet with a laser beam has a problem in that an excessive thermal stress accompanied with sharp temperature rise is applied to a vicinity of a region irradiated with the laser beam, and a problem in that a thermal residual strain occurs in vicinities of the fused end surfaces. In a case where influences of those problems are significant, the glass sheet may be deformed due to warpage or the like, or may break.
In this context, for example, Patent Literature 1 discloses the following. Specifically, after the glass substrate is preheated by the defocused laser beam, the glass sheet is fused with the laser beam focused at the micro point, and is then annealed again with the defocused laser beam. In this manner, a thermal strain is reduced.
Note that, in Patent Literature 1, similarly to the fusing laser beam, the annealing laser beam is vertically radiated directly from above onto the glass sheet.
Further, in Patent Literature 1, the output end of the preheating laser beam, the output end of the fusing laser beam, and the output end of the annealing laser beam (laser irradiators) are arrayed above the preset cutting line of the glass substrate, and the regions to be irradiated with the respective laser beams are spaced apart from one another at certain intervals.